1. Field of the Invention
The present invention relates to various optical devices exemplarily used for optical communications and, more specifically, an optical device such as a dispersion compensator for polarized waves and wavelengths of an optical fiber, an optical isolator, an optical modulator, and a photonic sensor used for detecting voltage or electric current flowing through a power transmission line or a power distribution line.
2. Description of the Background Art
Conventional various optical devices are first described below. FIG. 31 is a schematic diagram illustrating the structure of an optical isolator, which is one example of the conventional optical devices. The optical isolator includes a first and second lenses 1003 and 1004 for coupling a first optical fiber 1001 and a second optical fiber 1002 to each other through an optical system. Placed between these lenses are a polarizer 1005, a Faraday device 1006, and an analyzer 1007. Note that, in FIG. 31, outer lines of a light beam going through the optical system is represented as straight lines. Furthermore, there exists a magnetic field 1008 in the optical isolator enough to rotate the plane of polarization. Also, the polarizer 1005 and the analyzer 1007 form an angle of 45 degrees. The Faraday device 1006 is exemplarily implemented by a garnet crystal.
Described next is the principle of the optical isolator. In the optical isolator shown in FIG. 31, unpolarized light emitted from the first optical fiber 1001 is coupled through the first lens 1003 to the polarizer 1005, and therein converted into linearly polarized light. Then, the linearly polarized light goes to the Faraday device 1006 that rotates a plane of polarization thereof by 45 degrees. The linearly polarized light with its plane of polarization rotated is coupled by the analyzer 1007 having the above stated angle to the second optical fiber 1002 through the second lens 1004.
On the other hand, return light from the second optical fiber 1002 is coupled through the second lens 1004 to the analyzer 1007 for conversion into linearly polarized light. Then, the linearly polarized light goes to the Faraday device 1006 that rotates a plane of polarization thereof by 45 degrees. In the analyzer 1005, however, the plane of polarization of the linearly polarized light is perpendicular to the polarizing direction of the polarizer 1005. Therefore, no return light can be coupled to the first optical fiber 1001 through the first lens 1003. As such, the conventional optical isolator requires two lenses for coupling optical fibers.
Described next is a conventional dispersion compensator. In the conventional dispersion compensator, an optical system is placed between optical fibers. Thus, for coupling therebetween the optical system, at least two lenses are required.
Described next is a conventional optical modulator. The optical modulator functionally includes, for example, a polarizer, a xcex/4 plate, a Pockels device, and an analyzer. Linearly polarized light obtained by the polarizer becomes circularly polarized light by the xcex/4 plate, and then becomes elliptically polarized light depending on the electric field applied to the Pockels device. In the analyzer, this elliptic polarization causes changes in the amount of light. Thus, optical modulation can be achieved depending on the applied electric field. Such conventional optical modulator also requires at least two lenses for coupling the optical system between optical fibers.
The structure of the conventional optical modulator is described in more detail. For example, as shown in FIG. 32, a Mach-Zehnder type modulator 2012 used as the optical modulator is formed on a substrate 2001 made of LiNbO3 crystal, for example. In this Mach-Zehnder type modulator 2012, a waveguide unit 2002 includes a waveguide supplied at its incidence side with unpolarized light (TM light+TE light) 2005, and waveguides each polarizing and separating the unpolarized light into two polarized lights (TM light and TE light) for emission, and a waveguide coupling these lights for emission. Among these waveguides, the waveguides for polarization and separation are provided with electrodes 2003 to one of which a predetermined electric field is applied by a signal source 2004. Output light 2010 is coupled to an optical fiber 2006 through a lens 1009. The optical fiber 2006 is composed of a core 2007 through which light is transmitted, and a clad 2008.
As stated above, the conventional optical device such as the optical modulator requires expensive waveguides and at least one lens for optically coupling the waveguides and the optical fibers. Moreover, such coupling requires enormous amount of time and effort.
Described next is a conventional optical sensor. FIG. 33 is a schematic front perspective view of one conventional optical voltage sensor. This optical voltage sensor includes a sensor part, a light-emitting part, a light-receiving part, and signal processing circuits in light-emitting and light-receiving sides (not shown). The sensor part is composed of a polarizer 241, a 1/4 waveplate (also called xe2x80x9cxcex/4 platexe2x80x9d) 242, an electro-optic crystal 243, and an analyzer 244, all arranged on the same optical axis in such order from a light incidence side as mentioned above. The light-emitting part includes an E/O circuit including a light-emitting device typified by LED (Light Emitting Diode) as a light source, and an incidence side optical system composed of an optical fiber 246a, a ferrule 248a, a GRIN lens 247a, and a holder 245a, all of these arranged on the same optical axis and attached together on each optical axis plane with an adhesive. The light-emitting part includes an output side optical system composed of an optical fiber 246b, a ferrule 248b, a GRIN lens 247b, and a holder 245b, all of these arranged on the same optical axis and attached together on each optical axis plane with an adhesive, and an O/E circuit including a device for converting an optical signal emitted from the output side optical system into an electrical signal.
In the sensor part of the above optical voltage sensor, the polarizer 241, the xcex/4 plate 242, the electro-optic crystal 243, and the analyzer 244 all arranged on the same optical axis are attached together on each optical axis plane with an adhesive. Here, the optical axis plane is a plane perpendicular to the optical axis. Each of these optical components has two such planes: an plane of incidence and a plane of emittance. On the electro-optic crystal 243, a pair of electrodes 235 is evaporated, and electrically connected to a pair of electrode terminals 249 by lead wires. Between the electrode terminals 249, voltage to be measured by this optical voltage sensor is applied.
The signal processing circuits in the light-emitting and light-receiving sides are respectively connected through the light-emitting part and the light-receiving part to the sensor part. In the sensor part, the polarizer 241 is fixed, with an adhesive, at its plane of incidence to the optical axis plane of the GRIN lens 247a in the light-emitting part. The analyzer 244 is fixed, with an adhesive, at its plane of emittance to the optical axis plane of the GRIN lens 247b. The adhesively fixed sensor part, incidence side optical system in the light-emitting part, and output side optical system in the light-receiving part are mechanically fixed to a case (not shown). As the adhesive for the optical components in the above optical voltage sensor, epoxy resin or urethane resin is used.
In the above optical voltage sensor, used as the electro-optic crystal 243 is Bi12SiO20 (BSO), KH2PO4 (KDP), or a natural birefringent material such as LiNbO3 and LiTaO3, for example.
With reference to FIG. 34, the operational principle of the optical voltage sensor is described next. When an LED whose center wavelength is 0.85 xcexcm is exemplary used as the light source in the light-emitting part, unpolarized light emitted therefrom is inputted as incident light 109 to the sensor part. This incident light 109 passes through the polarizer 241 of the sensor part, and then becomes linearly polarized light. This linearly polarized light passes through the xcex/4 plate 242 to become circularly polarized light, and then passes through the electro-optic crystal (LiNbO3) 243 to become elliptically polarized light depending on applied voltage Vm to the electro-optic crystal (LiNbO3). That is, the polarization state of the elliptically polarized light in the electro-optic crystal 243 is varied depending on the applied voltage Vm. Such elliptically polarized light passes through the analyzer 244, and then is received as output light 110 by the light-receiving part. The intensity of the output light 110 is varied depending on the polarization state of the elliptically polarized light in the electro-optic crystal 243, which is varied according to the applied voltage Vm, as stated above. Therefore, by monitoring, at the light-receiving part, the change in the output intensity of the analyzer 244 to calculate a modulation index of the amount of light (intensity), the applied voltage Vm can be measured. Here, the modulation index of the amount of light is a ratio of AC components to DC components in the amount of light.
The light voltage sensor is often used outdoors under a hostile environment, and therefore required to have such temperature dependency as that change in modulation index at xe2x88x9220xc2x0 C. to 80xc2x0 C. is preferably below xc2x11%. Such temperature dependency is caused by changes in refractive index due to stress at an adhesive portion on the xcex/4 plate 242 and the electro-optic crystal 243, or by temperature dependency of birefringence of the xcex/4 plate 242. Also, when the electro-optic crystal 243 having natural birefringence such as LiNbO3 is used, the output of the optical voltage sensor is varied, for example, depending on the beam state of the incident light coming to the electro-optic crystal 243.
FIG. 35 is a graph exemplarily illustrating a relation between an angular deviation xcex1 and a directional deviation xcex2, and the output of the optical voltage sensor. In FIG. 35, xcex21 represented by a dotted line indicates outputs when the directional deviation are 0, 90, 180, and 270 (degrees). xcex22 represented by a one-dot-chain line indicates outputs when the directional deviation are 45 and 225 (degrees). xcex23 represented by a two-dot-chain line indicates outputs when the directional deviation are 135 and 315 (degrees). As shown in FIG. 35, depending on the beam state of the incident light coming to the electro-optic crystal 243 (the angular deviation xcex1 and the directional deviation xcex2), the output of the optical voltage sensor, that is, the modulation index, is varied, and the temperature dependency thereof are varied.
To cope with the above problems, the following three methods have been suggested for improving the temperature dependency.
(1) A first method, disclosed in Japanese Patent Laid-Open Publication No. 9-145745 (1997-145745), is to improve the temperature dependency of the electro-optic crystal by relaxing stress applied thereto. This relaxation is achieved by fixing the electro-optic crystal without an adhesive.
(2) A second method, disclosed in Japanese Patent Laid-Open Publication No. 3-44562(1991-44562), is to improve the temperature dependency of natural birefringence of the electro-optic crystal by reducing angular deviation of the incident light to 0.2xc2x0 or less by improving surface accuracy of each optical component.
(3) A third method, disclosed in Japanese Patent Laid-Open Publication No. 7-248339 (1995-248339), is to improve the temperature dependency of the sensor output by an incident angle adjuster changing incident angle of the incident light to the electro-optic device depending on the ambient temperature. In the incident angle adjuster, output changes due to temperature change are cancelled out with output changes due to incident angle change.
As stated above, in the conventional optical device, at least one (or two) lens(es) are required for connecting the optical system between optical fibers, thereby increasing the number of components. Moreover, such coupling of the optical system requires enormous amount of time and efforts. Therefore, with the above mentioned structure, the optical device disadvantageously costs more.
Furthermore, the optical sensor bears another unique problems in relation to the temperature dependency. That is, according to the first method, fluctuations in beam state that cause large temperature dependency can be prevented, but variations in temperature dependency cannot be prevented if the initial beam state fluctuates. The second method is easy to use, but axial deviation affects not only angular deviation, but also directional deviation. Therefore, only reducing axial deviation based on the second method do not yield stable dependency. In the third method, the incident angle adjuster for changing the incident angle of the incident light to the electro-optic crystal depending on ambient temperature is required. This causes complexity in structure, leading to reduction in productivity and increase in cost. Also, as stated above, axial deviation affects not only angular deviation, but also directional deviation. Therefore, only adjusting the incident angle of the light, that is, the axial deviation, does not yield stable temperature dependency.
To cope with the above problems, the Applicant has submitted an application of Japanese Patent Laid-Open Publication No. 11-215798 (1996-215798) disclosing the invention of an optical voltage sensor based on a method of controlling the modulation index by using axial deviation characteristics of an electro-optical crystal having natural birefringence. According to the optical voltage sensor of the above pending application, the temperature dependency of the optical voltage sensor is improved by controlling the beam state of the incident light to the electro-optical crystal. That is, by appropriately setting the state of axial deviation in consideration of not only angular deviation, but also directional deviation, the temperature dependency is improved.
However, controlling the beam state based on the invention of the above pending application requires beam-state management for preventing variations in beam state caused by tolerances among optical components such as a lens, thereby disadvantageously increasing cost.
Therefore, to bring down the price of optical voltage sensors, the beam state has to be managed at low cost.
Therefore, an object of the present invention is to provide a low-cost optical device having a smaller number of components capable of easily coupling an optical system between optical fibers at low cost. A further object of the present invention is to provide an optical sensor such as an optical voltage sensor with temperature dependency stabilized by suppressing variations in beam state caused by tolerances among optical components and other factors.
The present invention has the following features to attain the above objects.
A first aspect of the present invention is directed to a method of fabricating a photonic crystal, including the step of forming the photonic crystal directly on an end surface of at least one optical fiber as a substrate. For example, a plurality of optical fibers are tied in bundle with each end surface aligned on a same plane to form an optical fiber bundle, and the photonic crystal directly is directly on an end surface of the optical fiber bundle formed by the end surfaces of the optical fibers aligned on the same plane as the substrate. Then, by separating the optical fiber bundle into the optical fibers, the photonic crystal formed on the each end surface of the optical fibers is obtained.
A second aspect of the present invention is directed to a method of fabricating a photonic crystal. The method includes the step of forming the photonic crystal by making, in a predetermined section along an optical axis of an optical fiber composed of a core through which light propagate and a clad surrounding the core, a plurality of columns penetrate through the core. For example, the clad is partially removed, in the predetermined section from the optical fiber to form at least one plane parallel to the optical axis, and a plurality of holes penetrating the core are formed perpendicularly to the plane formed in the removing step.
A third aspect of the present invention is directed to an optical transmission member for transmitting light having a predetermined wavelength. The optical transmission member includes an optical fiber for transmitting the light inputted at one end surface thereof to another end surface thereof for output; and a photonic crystal layer formed on at least either one of the end surfaces of the optical fiber and functioning as a linear polarizer for the light having the wavelength.
A fourth aspect of the present invention is directed to an optical transmission member for transmitting light having a predetermined wavelength. The optical transmission member includes an optical fiber for transmitting the light inputted at one end surface thereof to another end surface thereof for output; and a photonic crystal layer formed on at least either one of the end surfaces of the optical fiber and functioning as a xcex/4 plate for the light having the wavelength.
A fifth aspect of the present invention is directed to the optical transmission member for transmitting light having a predetermined wavelength. The optical transmission member includes an optical fiber for transmitting the light inputted at one end surface thereof to another end surface thereof for output; and a photonic crystal layer formed on at least either one of the end surfaces of the optical fiber and functioning as a photonic-crystal circular polarizer for the light having the wavelength.
A sixth aspect of the present invention is directed to an optical device at least one functional part formed as the photonic crystal with a plurality of columns penetrating through a core in a predetermined section of an optical fiber along an optical axis of the optical fiber, and a propagation part for propagating the light as a function of the optical fiber. The functional part may be formed by the plurality of columns parallel to each other and periodically distributed on a plane perpendicular to a longitudinal direction of the columns. The plurality of columns forming the functional part may penetrate through the core and the clad of the optical fiber. Also, the plurality of columns forming the functional part may have a refractive index different from a refractive index of material forming the core. All or part of the plurality of columns forming the functional part may be a hole, or made of material having a Faraday effect or material having an electro-optic effect.
Furthermore, electrodes may be provided on a surface formed by partially removing the clad. The electrodes may be provided in pair on a surface perpendicular to a longitudinal direction of the plurality of columns forming the functional part. Alternatively, the electrodes may be provided in pair on two surfaces parallel and opposed to each other with the functional part therebetween, and perpendicular to the longitudinal direction of the plurality of columns forming the functional part. Still alternatively, the electrodes may be provided in pair on two surfaces parallel and opposed to each other with the functional part therebetween, and parallel to the optical axis and the longitudinal direction of the plurality of columns forming the functional part. Still alternatively, the electrodes may be arranged to apply the electric field to the functional part in a direction parallel to the optical axis of the optical fiber. Still alternatively, the electrodes may be arranged to apply the electric field to the functional part perpendicularly to a longitudinal direction of the plurality of columns forming the functional part and the optical axis of the optical fiber. Still alternatively, the electrodes may be arranged to apply the electric field to form a predetermined angle with a longitudinal direction of the plurality of columns along a plane perpendicular to the optical axis.
The functional part may include a first functional part composed of a plurality of columns parallel to each other and periodically distributed on a plane perpendicular to a longitudinal direction of the columns, the columns made of a Faraday crystal having a refractive index different from a refractive index of material forming the core; and a second functional part composed of a plurality of holes parallel to each other and distributed on a plane perpendicular to a longitudinal direction of the holes. The longitudinal direction of the plurality of columns forming the first functional part may form an angle of 45xc2x0 with the longitudinal direction of the holes forming the second functional part along a plane perpendicular to the optical axis.
The first functional part may be composed of a plurality of columns made of an electro-optic crystal. The second functional part may be composed of a plurality of first holes. The longitudinal direction of the columns may be perpendicular or parallel to the longitudinal direction of the first holes along a plane perpendicular to the optical axis.
The functional part may further include a third functional part composed of a plurality of second holes. The longitudinal direction of the columns may be perpendicular or parallel to the longitudinal direction of the second holes along the plane perpendicular to the optical axis.
Furthermore, the functional part may be formed as a photonic crystal with a predetermined refractive index and state of distribution, to have a wavelength dispersion characteristic of recovering a waveform of the light to be a steep waveform for output, the light being spread by a wavelength dispersion characteristic unique to an optical fiber through which the light passed before inputted to the optical fiber.
The present optical device may further include a guide for surrounding the optical fiber. The guide may be cylindrically shaped having a diameter approximately equal to a diameter of a ferrule of another optical fiber connected to the optical fiber. By way of example only, such guide is a capillary.
A seventh aspect of the present invention is directed to an optical isolator that includes first and second optical fibers formed by a plurality of holes parallel to each other penetrating through the core in a predetermined section along an optical axis and periodically distributed on a plane perpendicular to a longitudinal direction of the holes; a Faraday device placed to be closely attached between the first and second optical fibers; and a guide for mechanically adjusting an optical axis of the first optical fiber and an optical axis of the second optical fiber. In the optical isolator, a longitudinal direction of the holes of the first optical fiber forms an angle of 45xc2x0 with a longitudinal direction of the holes of the second optical fiber along a plane perpendicular to the optical axis.
An eighth aspect of the present invention is directed to an optical sensor that includes a light-emitting part for emitting a light beam; a sensor part including circular polarizer means for converting unpolarized light into circularly polarized light, an electro-optic crystal film, and an analyzer sequentially arranged on a predetermined optical axis set along an optical path of the light beam; and a light-receiving part for receiving the light beam after passing through the sensor part. The optical sensor measures, based on the light beam received by the light-receiving part, a voltage applied to the electro-optic crystal film. In the optical sensor, the light-emitting part includes a first optical fiber for inducing the light beam into the sensor part. The light-receiving part includes a second optical fiber for inducing, from the sensor part, the light beam after passing therethrough. The circular polarizer means includes a polarizer for converting the unpolarized light into linearly polarized light; and a xcex/4 plate for converting the linearly polarized light into the circularly polarized light. Here, the polarizer is formed on an end surface of the first optical fiber as a photonic crystal layer for converting the light beam from the light-emitting part into a linearly polarized beam. The analyzer is formed on an end surface of the second optical fiber as a photonic crystal layer for converting the light beam after passing through the sensor part into a linearly polarized beam.
A ninth aspect of the present invention is directed to an optical sensor that includes a light-emitting part for emitting a light beam; a sensor part including circular polarizer means for converting unpolarized light into circularly polarized light, an electro-optic crystal film, and an analyzer sequentially arranged on a predetermined optical axis set along an optical path of the light beam; and a light-receiving part for receiving the light beam after passing through the sensor part. The optical sensor measures, based on the light beam received by the light-receiving part, a voltage applied to the electro-optic crystal film. In the optical sensor, the light-emitting part includes a first optical fiber for inducing the light beam into the sensor part. The light-receiving part includes a second optical fiber for inducing, from the sensor part, the light beam after passing therethrough. Here, the circular polarizer means is formed on an end surface of the first optical fiber as a photonic crystal layer for converting the light beam from the light-emitting part into a circularly polarized beam. The analyzer is formed on an end surface of the second optical fiber as a photonic crystal layer for converting the light beam after passing through the sensor part into a linearly polarized beam.
A tenth aspect of the present invention is directed to an optical sensor that includes a light-emitting part for emitting a light beam; a sensor part including a polarizer, a magneto-optic crystal film, and an analyzer sequentially arranged on a predetermined optical axis set along an optical path of the light beam; and a light-receiving part for receiving the light beam after passing through the sensor part The optical sensor measures, based on the light beam received by the light-receiving part, a voltage applied to the magneto-optic crystal film. In the optical sensor, the light-emitting part includes a first optical fiber for inducing the light beam into the sensor part. The light receiving part includes a second optical fiber for inducing the light beam for the sensor part. Here, the polarizer is formed on an end surface of the first optical fiber as a photonic crystal layer for converting the light beams from the light-emitting part into a linearly polarized light beam. The analyzer is formed on an end surface of the second optical fiber as a photonic crystal layer for converting into the light beams after passing through the sensor part into a linearly polarized beam.
An eleventh aspect of the present invention is directed to an optical sensor that includes a light-emitting part for emitting a light beam; a sensor part including a polarizer, a xcex/4 plate, an electro-optic crystal, and an analyzer sequentially arranged on a predetermined optical axis set along an optical path of the light beam; and a light-receiving part for receiving the light beam after passing through the sensor part. The optical sensor measures, based on the light beam received by the light-receiving part, a voltage applied to the electro-optic crystal. The optical sensor includes a first reflective film having a reflection plane perpendicular to the optical axis and placed between the xcex/4 plate and the electro-optic crystal; and a second reflective film having a reflection plane perpendicular to the optical axis and placed between the electro-optic crystal and the analyzer. An interval between the first reflective film and the second reflective film is an integer multiple of half a wavelength of the light beam.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.